I. Field
The following description relates generally to communication systems, and more particularly to a method and apparatus for imbalance compensation in wireless networks.
II. Background
In a communication network, a mobile device communicates with other mobile devices through various base transceiver stations (BTSs). A BTS is an equipment that receives data from one mobile device and transmits it to another mobile device, thereby facilitating the communication between them. Mobile devices are also referred to as access terminals (ATs). Each BTS serves a pre-defined geographical area, which is divided into smaller areas called sectors. Therefore, each sector covers a geographic area. The AT can select a suitable sector within its geographic vicinity and establish a communication link with the BTS of the selected sector. The link then facilitates the communication between the AT and other ATs. A base station controller (BSC) controls and monitors various BTSs and ATs and helps in the selection of a suitable sector.
The process of selecting a sector by the AT depends on the signal strength the sector provides to the AT. The data rate control (DRC) value of the sector is a measure f the strength of the forward link (FL). The strength of the FL relates to the data rate that can be supported by the current radio frequency (RF) conditions of the communication link between the AT and the BTS of the sector. The AT typically selects the sector that has the highest DRC value. The selected sector then transmits the data on the FL.
In the Evolution-Data Optimized (EV-DO or, simply, DO) family of protocols, there is no single reverse link (RL) serving sector. Instead, every BTS in an AT's active-set participates in the decoding process. This is known as RL soft handoff. To maintain the decoding performance at a desired level, each BTS instructs the AT, through a one-bit command known as the reverse power control (RPC), to either increase or decrease its pilot power by an offset. Since the AT needs to be decoded by no more than one BTS, it only increases its pilot power if all RPC commands it receives instruct it to do so. This approach allows for minimizing both the AT's transmission power and the interference it causes. To control the load, which is the total received power, a BTS broadcasts an indicator known as a reverse activity bit (QRAB) whenever its received power exceeds a preset threshold. The AT's receiving the QRAB react by reducing the gain of their RL traffic channel relative to the pilot channel. This reduces the AT's contribution to the BTS's load. However, it also forces the AT to lower its RL transmission rate to maintain the decoding performance at the BTS.
Once an AT is powered on, it continuously searches for new BTS's and adds them to the AT's active-set, provided that the received Ecp/I0 exceeds a threshold called Tadd. Ecp/I0 is the difference between the signal strength and the noise floor and a measure of the portion of the RF signal which is usable. On the other hand, the AT drops a BTS from its active-set, once its received Ecp/I0 falls below another threshold called Tdrop. Usually, Tdrop is a few dB smaller than Tadd to provide some hysteresis; however, in this disclosure it is assumed that the two thresholds have the same value for simplicity (basic results of the disclosure follow in either case). An AT normally picks the pilot in its active-set with the largest Ecp/I0 as its FL serving sector. However, if this BTS resets the DRC lock bit corresponding to the AT, then the AT will proceed to use the pilot from the BTS having the next largest Ecp/I0. A BTS may reset the DRC lock bit of an AT if, for example, the BTS is performing poorly in decoding the AT's OH channels.
When there is heterogeneity in DO networks, where heterogeneity refers to the different BTS capabilities, such as transmission power, imbalance can occur. For example, f-Imbalance and OH-Imbalance are two types of imbalances that may be caused by heterogeneity. For example, OH-Imbalance, which is also referred to as Pilot\DRC\ACK imbalance, occurs when an AT is served on the FL by a farther-away high power sector, while the AT is power controlled on the RL by a nearby low power sector. The pilot and\or overhead (e.g., DRC or ACK) channels, which are transmitted on the RL and are essential for proper operation of the FL, may be too weak to be received at the high power sector. Also, as another example, f-Imbalance occurs when an AT is closer to a cell that is not power-controlling the AT.
Consequently, it would be desirable to address one or more of the imbalances or other deficiencies described above.